The present invention relates to a method for producing a filter cake created in filtering. By means of said method, there is achieved a reduction in the residual moisture of the solids cake created particularly in a suction drier, and a control of the thickness of said solids cake.
From the FI patent 61,739, there is known a drying method and apparatus for drying a web-like, pulverous, solid or porous material, in which method the target to be dried is put, by intermediation of a finely divided, liquid-saturated suction surface, to hydraulic contact with the liquid maintained at under pressure with respect to the target to be dried. The apparatus employed in said method comprises a fine porous liquid suction surface, where the radii of the fine pores are mainly within the range of 0.5-2 micrometers.
The fine porous liquid suction surface constitutes the filter surface of the suction drier filter medium, which filter surface is put into contact with the material to be dried, such as slurry. While the filter surface is for instance plane-like, and by creating under pressure on the opposite side with respect to the filter surface of the filter medium, on the filter surface there is formed, owing to the influence of suction, a cake filtered of the material to be dried, which cake is then scraped off the filter surface. In connection with the making of the cake, the porous structure of the filter surface and the filter medium forming the filter surface is gradually filled with the finely divided material to be dried.
Various methods are developed for cleaning the filter surface of a suction drier provided with a fine porous liquid suction surface, for instance the sequential and continuous ultrasonic cleaning methods described in the FI patents 76,705 and 82,388, respectively, as well as the reverse pressure washing method described in the FI patent 87,539. However, attention has not been paid to the creation of the cake itself, which is formed of the finely divided material to be dried, but the cake has been allowed to form essentially freely after switching, on the under pressure, when the filter surface has been cleaned. In order to facilitate rapid cake formation and to maintain the required capacity, the under pressure has been allowed to prevail, starting from the very beginning of the process, at the same level throughout the whole cake formation process. At the beginning of the cake formation, cake is formed essentially rapidly on the cleaned filter surface, wherefore the cake layer created against the filter surface becomes dry and compact owing to effective suction. Now the porous structure of the filter surface is blocked, and further problems related to cake formation arise, affecting, among others, the residual moisture of the cake and the thickness thereof, as well as having their influence on whether the cake remains on the filter surface until the cake removal stage proper begins. A common solution to the problems caused by the thickness and residual moisture of the filter cake has been to shorten the cake formation period by lowering the surface of the slurry contained in the filtering device. The lowering of the slurry surface may prevent the sequential and continuous ultrasonic cleaning of the filter surface, described for instance in the FI patents 76,705 and 82,388.
Consequently, the object of the present invention is to control the properties of the filter cake, produced by a suction drier provided with a fine porous liquid suction surface, during the cake formation process itself, so that the residual moisture and thickness of the cake can be advantageously controlled without essentially lowering the surface of the slurry contained in the filtering device. The essential novel features of the invention are apparent from the appended claims.
In a method according to the invention, the properties of the filter cake, produced by a suction drier provided with a fine porous liquid suction surface, are advantageously controlled by adjusting the compactness of the filter cake, so that in connection with the cake formation, the applied pressure difference between the side on the filter surface of the filter medium and the side opposite to the filter surface is controlled in order to advantageously alter the cake formation speed and to maintain the slurry surface on a level which allows the use of at least one filter surface cleaning member, and when necessary, substantially throughout the whole cake formation period. Thus at least one of the desired filter cake properties, either residual moisture or thickness, can be improved in order to facilitate the further processing of the cake and the cleaning of the filter surface.